Ocean Acidification Facts and Discussion

The pH Scale

The pH scale (see "A primer on pH") measures how acidic or basic a substance is. It ranges from 0 to 14. On this scale 7 is neutral, numbers lower than 7 are acidic, and numbers higher than 7 are basic. Since the Industrial Revolution, the average pH of global surface ocean waters has decreased by 0.11 units, from 8.21 to 8.10. This may not seem significant, but the pH scale is logarithmic, meaning a decrease of a single pH unit represents a ten-fold increase in acidity. The 0.1 drop in pH that we have seen corresponds to a 30% increase in ocean acidity.

What Is Ocean Acidification?

Ocean acidification is often called the “other carbon dioxide problem” because, like climate change, it is primarily caused by CO2 emissions. Over the past 150 years, since the Industrial Revolution, there has been a significant increase in the amount of CO2 in the atmosphere. Concentrations of CO2 in the air and in the ocean maintain a certain balance. As a result, when concentrations of CO2 increase in the air, the ocean absorbs some of that CO2 to regain the balance. The ocean is currently taking up approximately 25% of all CO2 released into the atmosphere.

This absorption of CO2 into the oceans has moderated warming of the atmosphere caused by increasing CO2 concentrations. However, it has also altered, and will continue to alter, the chemistry and biology of the oceans. As we continue burning fossil fuels, the concentration of CO2 in the atmosphere will continue to rise, and the ocean will continue to react by absorbing some of this CO2.

Scientists have estimated that by the year 2100, if we operate on a “business as usual” CO2 emissions scenario, the pH of ocean surface waters could decline by 0.3 to 0.4 units. This would represent a 150% increase in acidity since pre-industrial times. Some scientists say that these changes are occurring at a rate at least ten times faster than any other time over the past 50 million years. This is worrisome because some calcifying marine organisms, such as crabs and mollusks, have a more difficult time forming calcium carbonate shells and skeletons in lower pH waters. These organisms are important because they play a significant role in shaping marine ecosystems, and are essential food sources for commercial, subsistence, and recreational species.

The chemistry of the ocean has fluctuated in the past, but the rate at which it is changing today could be too quick for organisms to adapt to in order to survive. Although the chemical effects of ocean acidification are well understood, scientists are just beginning to get a grasp on some of the potential biological implications. Recent research emphasizes the need for increased monitoring and understanding of ocean acidification and its effects on marine ecosystems. Ocean acidification has far-reaching and potentially devastating ramifications for many species, and raises serious concerns about the future of our oceans.

The Chemical Process of Ocean Acidification

When atmospheric carbon dioxide (CO2) is absorbed into the ocean it reacts with water molecules (H2O) and forms a weak acid, called carbonic acid (H2CO3). When acids dissolve in water they break apart. In the case of carbonic acid, it breaks into hydrogen ions (H+) and bicarbonate ions (HCO3-). Some of the hydrogen ions remain as hydrogen ions, which lowers pH and makes the water more acidic. Many of the hydrogen ions, however, combine with carbonate ions (CO32-), which are present in the oceans, to form more bicarbonate ions. In addition, CO2 combines directly with water and carbonate molecules to form bicarbonate ions.

These reactions reduce the pool of carbonate ions in the ocean. Shells of marine organisms are primarily made up of calcium carbonate (CaCO3), which is formed when calcium ions (Ca2+) combine with carbonate ions (CO32-). Carbonate ions, however, bind more easily to the hydrogen ions released when CO2 dissolves in the oceans. This makes shell formation more difficult, because there are fewer carbonate ions available to shell-forming organisms. In summary, increased CO2 in the oceans increases the acidity of the water and decreases the availability of carbonate ions for shell-forming organisms.

consumption of carbonate ions impedes calcification This figure showing the chemical reaction is from the NOAA PMEL Carbon Program website.

Biological Impacts of Ocean Acidification

A great deal of current research has focused on the biological effects of decreased carbonate ions on organisms that form calcium carbonate shells and skeleton, such as calcareous plankton, crabs, clams, oysters, and corals. Increasing CO2 in the ocean causes a number of chemical reactions that lower the availability of carbonate ions. In more acidic water, calcifying organisms must exert more energy to form shells and skeletons, which can result in slower growth and thinner, abnormal shells, and leave them less energy to find food and reproduce. Any reduction in pH can make it harder for calcifying organisms to build and maintain their shells. With weaker shells, these organisms will be more vulnerable to injury and predation, resulting in reduced survival.

However this research is still relatively new, so there are a lot of gaps in our understanding of how ecosystems are likely to change. Some organisms that rely on CO2, such as photosynthetic seagrasses and algae, may benefit from higher CO2 concentrations, whereas many calcifying species have a more difficult time forming shells and skeletons in more acidic waters. Factors such as seawater temperature and community structure also affect responses. There does not seem to be a common pattern in how different species, and even different life stages of the same species, respond to ocean acidification, making predictions extremely difficult.

Species that rely on calcifying organisms for food or shelter may be negatively affected by ocean acidification if calcifying organisms begin to disappear. While some organisms may be able to adapt to rapid change, the rate at which ocean acidification is occurring may be too fast for others to adapt. Shifts in species composition of ocean communities are likely to happen, favoring species that can survive and compete better under more acidic conditions. These changes could have impacts on currently established commercial, recreational, and subsistence fisheries.

Beginning in 2005, some of the first evidence of ocean acidification was observed on the coasts of Washington and Oregon, where oyster hatcheries experienced near total failure of developing oysters. In 2012 scientists concluded that ocean acidification was at least partly responsible for the oyster larvae deaths. They stated that the larvae likely died because the corrosive, low pH waters [PDF; 1.7 MB] brought to the surface by upwelling (the wind-driven process of deep ocean water being brought to the surface) caused them to exert too much energy to build their shells. What is occurring on the West Coast has implications for the future if ocean acidification continues to increase, especially in places where upwelling is prevalent such as Alaska.

Ocean acidification can also affect non-calcifying species. Laboratory experiments have shown that lower pH can:

Note that these papers highlight only a few of many examples of how ocean acidification affects non-calcifying species, and more will likely be discovered as ocean acidification increases and more research is devoted to the topic.

Research on Ocean Acidification

In Alaska, researchers focus on monitoring ocean chemistry in the wild and in hatchery settings, tracking physiological responses of marine organisms to ocean acidification at multiple life stages, and predicting potential socioeconomic impacts of ocean acidification on important commercial species.

Scientists at the University of Alaska Fairbanks Ocean Acidification Research Center, with support from the State of Alaska, the National Oceanic and Atmospheric Administration (NOAA), and Alaska Ocean Observing System, use sampling research cruises and permanently moored buoys, along with carbon wave gliders, equipped with scientific instruments to measure changes in ocean chemistry. Laboratory experiments are being conducted on Alaska’s commercially important species, such as red king crab and Alaska pollock, to determine any detrimental effects on these species at various life stages.

Approximately half of all federally managed fisheries depend on coral reefs at some point in their life cycle. Alaska is thought to have some of the most abundant and diverse cold-water corals in the world. Cold-water corals form dense “coral gardens” that are similar to tropical shallow-water coral reefs. The extent to which ocean acidification will affect corals will likely depend on the species and their geographic location. Some corals can use bicarbonate instead of carbonate to build their skeletons. Others can handle a wide range of pH values. However, there is likely going to be a change in the species composition on coral reefs, shifting to corals that can tolerate lower acidity. The NOAA Coral Reef Conservation Program has more information about deep-sea corals.

Laboratory studies have shown that commercially important red king crabs and Tanner crabs had difficulty growing and molting in waters where the pH is 7.8, a level scientists think could be possible by 2100 in some scenarios. In even more acidified conditions with pH as low as 7.5, red king crabs could not survive. Survival rates for Tanner crabs also decreased with increasing pH, although they resisted the change a bit more than red king crabs.

Researchers around the state are beginning to investigate how changes in ocean productivity will impact Alaska communities, which communities are likely going to be the most sensitive to change, and if residents are aware of the potential risks. This human dimension is important because communities must begin to adapt and prepare for future changes in the ocean.

To learn more about the research on ocean acidification being conducted in Alaska and elsewhere, and to keep up to date on recent findings, visit these web pages:

What Can We Do About Ocean Acidification?

Below are actions that can help reduce CO2 emissions: